- limit ECC support to ec*-sha2-nistp256:
* DROPBEAR_ECC now provides only basic support for ECC
- provide full ECC support as an option:
* DROPBEAR_ECC_FULL brings back support for ec{dh,dsa}-sha2-nistp{384,521}
- update feature costs in binary size
Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>